Electric vehicles have been known including motors that independently drive the left and right wheels, which each consist of at least one of forward and rearward wheels (refer to Japanese Unexamined Patent Application Publication No. 2008-222070). Such a driving system is referred to as twin-motor system or left-and-right-independent-drive system and has been drawing attention for its independent control of the driving forces (torques) applied to the left and right wheels, which improves kinematic performance. For example, a vehicle can generate a yaw moment during cornering through a difference between the driving forces (torque difference) applied to the left and right wheels in addition to the steering angle, so as to improve the cornering performance of the vehicle.
In the driving systems described above, the torque demanded by the driver (demand torque) is distributed and generated to the left and right motors. Thus, there is a problem which the torque difference generated by the left and right motors becomes smaller as the demand torque becomes larger. This is because, in general, the motors installed in the vehicle output the substantial maximum power when the demand torque is maximum. Thus, as the demand torque becomes larger, the margin of the motor output becomes smaller.
That is, depending on the operation by the driver, the driving systems described above can probably achieve a driver's request, but may nullify the torque difference between the left and right wheels during cornering. This interferes with the improvement of the cornering performance. On the other hand, the torque difference between the left and right wheels can be maintained through a decrease in the output of one of the left and right motors. In such a case, the decrease in the total output of the motors causes an insufficient response to a driver's request.